1. Field of the Invention
The present invention relates to dielectric ceramics suited for forming insulating substrates that serve as skeletons for packages for containing semiconductors and for various wiring boards such as multi-layer wiring boards, and to a method of producing the same. In particular, the invention relates to dielectric ceramics having a low dielectric loss in high-frequency regions, that can be produced by the co-firing with a low-resistance conductor such as copper, gold or silver, and to a method of producing the same.
2. Description of the Prior Art
There have been widely used ceramic wiring boards having a wiring layer of a high-melting point metal such as tungsten or molybdenum formed on the surface or inside of an insulating substrate of an aluminous sintered product.
Accompanying the trend toward highly sophisticated information technology in recent years, the frequency band of signals applied to the wiring boards is shifting to ever high frequency regions. In the wiring board that must transfer signals of such high frequencies, the conductor forming the wiring layer must have a small resistance to transmit high-frequency signals without loss and the insulating substrate must have a small dielectric loss in the high-frequency regions.
However, the above-mentioned wiring layer formed on the conventional ceramic wiring board, i.e., the wiring layer formed of a high-melting point metal such as tungsten or molybdenum, has a large resistance and permits signals to propagate at a slow speed. Besides, the above-mentioned wiring layer is not capable of transmitting signals of frequencies not smaller than 1 GHz. It therefore becomes necessary to form a wiring layer using a low-resistance metal such as copper, silver or gold instead of tungsten or molybdenum. However, such low-resistance metals have low-melting points, and are not capable of forming a wiring layer by the co-firing with alumina.
In recent years, therefore, there has been developed a wiring board using an insulating substrate made of a so-called glass ceramics comprising a composite material of a glass and ceramics. The glass ceramics has a dielectric constant of as low as about 3 to about 7, is suited for transmitting high-frequency signals compared with alumina ceramics, and is obtained by the firing at a temperature as low as from 800 to 1000.degree. C., and has an advantage in that it can be co-fired with a low-resistance metal such as copper, gold or silver.
For example, Japanese Unexamined Patent Publication (Kokai) No. 240135/1985 proposes a wiring board obtained by co-firing a green sheet together with a wiring pattern of a low-resistance metal, the green sheet comprising a filler such as Al.sub.2 O.sub.3, zirconia or mullite and a zinc borosilicate glass.
Besides, Japanese Unexamined Patent Publication (Kokai) No. 298919/1993 proposes a glass ceramics obtained by precipitating mullite or cordierite as a crystal phase.
The above-mentioned conventional glass ceramics can be formed by the co-firing with a low-resistance metal, but has a defect of low coefficient of thermal expansion (about 3 to about 5 ppm/.degree. C.).
The wiring board has various electronic parts (e.g., chips of GaAs) mounted thereon and is used being mounted on a printed board such as mother board formed of an organic resin. The mounting is effected relying on the so-called brazing generating, however, thermal stress. Further, thermal stress generates between the wiring board and the printed board due to thermal hysteresis of when the semiconductor element is operated and stopped repetitively. Here, the chip such as GaAs has a coefficient of thermal expansion of from 6 to 7.5 ppm/.degree. C., the printed board has a coefficient of thermal expansion of from 12 to 15 ppm/.degree. C., and the above-mentioned glass ceramics has a coefficient of thermal expansion which is greatly different therefrom. In the wiring board having an insulating substrate formed of the conventional glass ceramics, therefore, the mounting portion is peeled or is cracked due to thermal stress at the time of mounting or due to thermal stress when the semiconductor element is operated and stopped repetitively as a result of the above-mentioned great difference in the coefficient of thermal expansion. Therefore, such wiring boards have a very low reliability concerning the mounting and are not satisfactory for being practically used.
Further, the conventional glass ceramics has a large dielectric loss in the high-frequency regions, and the wiring board equipped with the insulating substrate of the glass ceramics fails to exhibit satisfactory high-frequency characteristics and cannot be used as a high-frequency wiring board for handling high-frequency signals such as of microwaves and millimeter waves.